Shi Siqi, Yang Peiqi, van der Tol Christiaan
Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, PO Box 217, Enschede 7500 AE, the Netherlands.
Key Laboratory of Virtual Geographic Environment, Ministry of Education, Nanjing Normal University, Nanjing, China.
Heliyon. 2023 Jun 7;9(6):e16413. doi: 10.1016/j.heliyon.2023.e16413. eCollection 2023 Jun.
Knowledge of the dynamics of vegetation phenology is essential for the understanding of vegetation-climate interactions. Although the interest in phenology study is growing, vegetation phenology in Africa received far less attention compared to the Northern Hemisphere. Africa straddles the northern and southern hemispheres, and the climate has a clear latitudinal gradient, which facilitates the study of the interaction between phenology and climate. In this study, the latitudinal and longitudinal gradients and temporal trends of start of growing season (SOS), peak of growing season (POS), and end of growing season (EOS) were examined using long-term satellite dataset during 1982-2015. The latitudinal variations in these phenology metrics were larger in the northern than those in the southern Africa, especially from 6°N northwards to 16°N. The latitudinal variations in southern Africa had no clear patterns due to the more complex climate systems. For the longitudinal variation, the temporal trends in POS and EOS exhibited a gradient-decreasing rate in northern Africa. Over the period from 1982 to 2015, the overall trends of the phenology in Africa were 'later SOS', 'later POS', and 'later EOS'. The faster rate of delay in EOS than in SOS resulted in a prolonged length of growing season (LOS) with 0.50 days/year on average in northern Africa, while a slower rate of delay in EOS than in SOS resulted in a shorter LOS with -0.12 days/year in southern Africa. The prolonged LOS in northern Africa contributes to the increase in the yearly-averaged Normalized Difference Vegetation Index (NDVI) from 1982 to 2000. Nevertheless, the NDVI appeared to have reached saturation around the 2000s, although the LOS was still extending after 2000s. Overall, the findings of this study provide an overall view of the spatial and temporal patterns of land surface phenology in the African continent, and a necessary component for future studies on the response of phenology to climate.
了解植被物候动态对于理解植被与气候的相互作用至关重要。尽管物候研究的关注度在不断提高，但与北半球相比，非洲的植被物候受到的关注要少得多。非洲横跨南北半球，气候具有明显的纬度梯度，这有利于研究物候与气候之间的相互作用。在本研究中，利用1982 - 2015年的长期卫星数据集，研究了生长季开始（SOS）、生长季峰值（POS）和生长季结束（EOS）的纬度和经度梯度以及时间趋势。这些物候指标的纬度变化在非洲北部比南部更大，特别是从北纬6°向北到北纬16°。由于气候系统更为复杂，非洲南部的纬度变化没有明显规律。对于经度变化，北非POS和EOS的时间趋势呈现出梯度递减率。在1982年至2015年期间，非洲物候的总体趋势是“SOS推迟”、“POS推迟”和“EOS推迟”。EOS的延迟速度比SOS快，导致北非生长季长度（LOS）平均每年延长0.50天，而EOS的延迟速度比SOS慢，导致南非LOS缩短，每年缩短0.12天。北非LOS的延长导致1982年至2000年年平均归一化植被指数（NDVI）增加。然而，尽管2000年后LOS仍在延长，但NDVI在21世纪初似乎已达到饱和。总体而言，本研究结果提供了非洲大陆陆地表面物候的时空格局的整体视图，是未来物候对气候响应研究的必要组成部分。
